This invention relates to semiconductor switch devices (e.g. an NPN switching transistor or a power switching diode) having a switchable reversebiased p-n junction between a first region (such as a p-type base of the transistor or of the diode) of a first conductivity type and a second region (such as a collector or cathode) of an opposite second conductivity type. The invention further relates to methods of manufacturing such a semiconductor switch device.
United States patent specifications U.S. Pat. No. 3,507,714 and U.S. Pat. No. 3,710,203 disclose respectively a power switching transistor and a power switching diode, each of which has a p-type base (xe2x80x9cfirstxe2x80x9d region) forming a switchable p-n junction with a second region of an opposite second conductivity type. The second region is an n-type collector region in U.S. Pat. No. 3,507,714 and an n-type cathode region in U.S. Pat. No. 3,710,203. In both the transistor and the diode, the first region includes a high-doped zone having a higher doping concentration of the first conductivity type than a low-doped zone of the first region. In the manufacture of these switch devices, a semiconductor body portion having a substantially uniform doping concentration of the first conductivity type is provided to form the low-doped zone adjacent to the p-n junction, the body portion having a surface located opposite the p-n junction; and a doping step is carried out by doping the body portion over a part of its thickness with dopant characteristic of p-type conductivity through the surface so as to provide the high-doped zone with a doping concentration which decreases towards the low-doped zone. The whole contents of U.S. Pat. No. 3,507,714 and U.S. Pat. No. 3,710,203 are hereby incorporated herein, as reference material.
In the method disclosed in U.S. Pat. No. 3,507,714 the NPN transistor has its base region formed by a uniformly low-doped p-type body portion (wafer) into which the p-type high-doped zone (and also n-type emitter and collector regions) are diffused. This method dates to the late 1960s, i.e. several decades ago. Nowadays it is more conventional to form an NPN transistor from a uniformly low-doped n-type body portion (epitaxial layer) into which the p-type base region and n-type emitter region are diffused. United States patent specification U.S. Pat. No. 4,805,004 discloses a variant NPN transistor in which the p-type base region comprises a uniformly low-doped p-type body portion (epitaxial layer) on a uniformly low-doped n-type body portion (epitaxial layer) of the collector region. The whole contents of U.S. Pat. No. 4,805,004 are also hereby incorporated herein, as reference material.
It is an aim of the present invention to change the design and manufacture of semiconductor switch devices (such as, e.g. NPN switching transistors and power switching diodes) so as to permit an improvement of their switching behaviour, e.g. in terms of fall-time and energy dissipation during turn-off of the device.
According to one aspect of the present invention, there is provided a semiconductor switch device as set out in claim 1.
According to another aspect of the present invention, there is provided a method of manufacture as set out in claim 7.
As described hereafter in more detail, the inventors have discovered that the switching behaviour can be significantly improved by providing an additional zone with a doping concentration in accordance with the present invention, between the low-doped and high-doped zones of the first region. When the semiconductor switch device is being switched off, it appears that this additional zone provides a low-resistance path for extracting the remaining plasma which is mainly present in the middle of the uniformly low-doped zone towards the end of the discharge period. A significant reduction in fall-time and energy dissipation can be achieved by this means.
The first and second regions may be respective anode and cathode regions of a switching diode. Thus, the high-doped zone of the first region may be provided with an anode contact at the surface, and the second region may be provided with a cathode contact at a surface of the cathode region opposite the anode region.
The first and second regions may be respective base and collector regions of a bipolar transistor. Thus, the body portion of the first conductivity type may be overdoped over a part of its thickness with dopant characteristic of the second conductivity type adjacent to the surface to provide the transistor with an emitter region which forms a p-n junction with the high-doped zone of the base region. The emitter region and high-doped zone of the base region may be provided with respective emitter and base contacts at the surface. The second region may be provided with a collector contact at a surface of the collector region opposite the emitter and base regions.
The present invention is particularly advangeous for so-called xe2x80x9cp-basexe2x80x9d switching devices, i.e. in which the conductivity type of the first region (and hence its low-doped zone and additional zone) is p-type. The minority charge carriers in a p-type region are electrons which have a high mobility, and so a p-base device constructed in accordance with the invention can have fast switching characteristics. Holes have a much lower mobility than electrons, and so the provision of the low-doped zone on the p-type side of the p-n junction and the provision of the additional zone between the low-doped and high-doped zones of the p-type region permits efficient removal of the holes when switching off the p-base device. Thus, the first region is preferably p-type, and the body portion may be typically of p-type silicon. Boron is a well-established p-type dopant in silicon, and several known boron doping technologies are suitable for providing the desired doping profiles of the low-doped zone, the additional zone and the high-doped zone. Phosphorus and/or arsenic are suitable n-type dopants for regions of opposite conductivity type in silicon. The starting material may be a uniformally boron doped silicon substrate. The switch device may be manufactured without a need for epitaxial growth, by locally doping such a substrate material (a remaining portion of which forms the uniformally low-doped zone) with the respective dopants to form the various regions and zones.
Typically the maximum doping concentration of the additional zone of the first region is one or two orders of magnitude lower than the doping concentration of the high-doped zone. It may be at least an order of magnitude higher than the doping concentration of the low-doped zone. Generally the low-doped zone of the first region has such a low doping concentration as to be depleted by a depletion layer which extends, when the switch device is operated in an off state, from the reverse-biased p-n junction across the thickness of the low-doped zone and into the additional zone of the first region. Indeed, both the additional zone and the low-doped zone may be fully depleted when the switch device is operated in its off state close to its breakdown voltage. In this manner a high blocking voltage can be obtained in the off state of the switch device, while still retaining fast turn-off due to the inclusion of the additional zone. The thickness of the additional zone with its additional doping concentration may be maximised to reduce the voltage drop therein, and the thickness of the low-doped zone may be maximised to increase the breakdown voltage together with the decrease in doping concentration of the additional zone towards the low-doped zone. Typically the thickness of the low-doped zone and the additional zone are of the same order and typically greater than that of the higher-doped zone. Depending on the type of switching device, the conductivity type determining dopant concentration of the high-doped zone may typically exceed 1017 cmxe2x88x923, whereas that of the additional zone may be between 1014 cmxe2x88x923 and 5xc3x971015 cmxe2x88x923. Typically, the conductivity type determining dopant concentration of the uniformally low-doped zone may be at most 5xc3x971014 cmxe2x88x923 and is preferably less than 1014 cmxe2x88x923.
The multiple zone structure of the first region in accordance with the present invention may be combined with various measures to control the electric field in a peripheral area of the device, in the off-state of the device. Thus, for example, extension zones may be provided at both sides of the reverse-biased p-n junction so as to extend towards each other at a surface of the body in a manner similar to that disclosed in U.S. Pat. No. 4,805,004. An advantageous different measure is described below, in which the p-n junction is extended to the surface adjoined by the first region, and an annular field-relief region of the second conductivity type is present in the low-doped zone of the first region at a location between the additional zone of the first region and the extension of the p-n junction at said surface, so as to control the spread of the depletion layer from the p-n junction.